ResMed shares were down about 6 percent Friday morning after the San Diego medical equipment maker reported second quarter earnings below analyst expectations.

ResMed earned $86.6 million, or 60 cents per share, in the quarter ended Dec. 31, compared with $77.9 million, or 53 cents per share, in the same quarter of 2012. Revenue of $384.3 million was 2 percent higher than a year ago.

Shares traded at $43.94, down $2.94 at 11:07 Pacific Time.

Changes in Medicare reimbursement caused a "temporary distraction," in U.S. sales, Michael Farrell, chief executive of ResMed, said in a Thursday afternoon conference call.

ResMed's products help people with sleep-disordered breathing, such as sleep apnea. They include flow generators that push air into the lungs, and masks that deliver the air.

"Sales revenues from masks were flat year-on-year while flow generator sales revenue declined by 5 percent year-on-year," Farrell said in a call transcript provided by Seeking Alpha. "It's also of note that we were up against some very tough prior-year comparisons in both those categories."

The results reflected record revenue and income, said ResMed. But analysts had expected more. ResMed was projected to bring in revenue of $396.2 million, Piper Jaffray's David Clair said in a research note. Clair kept the company's rating of "overweight."

William Blair's Ben Andrew downgraded ResMed to "market perform," in response to the difficulties adjusting to Medicare's competitive bidding program. Domestic sales declined for the first time ever, Andrew wrote.

"While we believe the sleep disordered breathing market will stabilize over the next couple of quarters and then should reaccelerate, we will step to the sidelines until we at least see stabilization in ResMed’s results," he wrote.

Clair wrote that he anticipated such a resurgence with the launch of new products, such as the AirFit P10 mask, designed to be lighter and quieter than previous masks. He slightly reduced the target price for ResMed from $57 to $56 per share.